Hybrid vehicles commonly use at least two different energy conversion processes that can include a mechanical engine and an electric motor. And the associated transmission, or whether a transmission is used at all, sometimes depends on the particular drive configuration of the hybrid vehicle—parallel, serial, or mixed. One example of a transmission that can be used in a mixed hybrid is an electrically variable transmission (EVT).
EVT transmissions typically have at least one planetary gear set where the mechanical engine and the electric motor are operatively connected to different members of the planetary gear sets. Further, wet clutches are used to change speed ratios in the transmission, and hydraulic systems are in turn used to control the wet clutches. Hydraulic systems are complex, costly, and require many components including the particularly bulky and difficult to manufacture valve bodies.